1. Field of the Invention
The present invention relates to display devices, such as organic electroluminescence display devices, which include a display panel comprising an arrangement of a plurality of pixels.
2. Description of Related Art
Progress has been made in developing organic electroluminescence displays (hereinafter referred to as “organic LED displays”) in recent years. Use of organic LED displays, for example, in portable telephones is under study.
The methods of driving such organic LED displays include the passive matrix driving method wherein the scanning electrodes and the data electrodes are used for time division driving, and the active matrix driving method wherein each pixel is held luminescent for one vertical scanning period. Furthermore, the methods of driving the organic LED display devices of the active matrix driving method include the display devices of the analog drive type wherein current of a magnitude corresponding to the data voltage is supplied to an organic EL element to turn on the EL element with a brightness corresponding to the data voltage, and the display devices of the digital drive type in which a multi-level gradation is produced by supplying to an organic EL element a pulse current having a duty ratio in accordance with the data voltage (e.g., JP-A No.312173/1998).
The present applicants have proposed the organic LED display devices of the digital drive type having a display panel comprising an arrangement of pixels 31 having a circuit structure shown in FIG. 15. With the organic LED display devices, each pixel 31 is provided with an organic EL element 30, a drive transistor TR2 for effecting or interrupting passage of current through the organic EL element 30 in response to input of an on/off control signal to a gate, a write transistor TR1 which is brought into conduction in response to the application of scanning voltage by a scanning driver, a capacitance element C to be supplied with a data voltage from the data driver by the write transistor TR1 conducting, and a comparator 32 having a pair of positive and negative input terminals to be supplied with the ramp voltage from the ramp voltage generating circuit and the output voltage of the capacitance element C for comparing the two voltages. The output signal of the comparator 32 is fed to the gate of the drive transistor TR2. The drive transistor TR2 has a source connected to a current supply line 33 and a drain connected to the EL element 30. The data driver is connected to one electrode (e.g., source) of the write transistor TR1. The other electrode (e.g., drain) of the write transistor TR1 has connected thereto one end of the capacitance element C and an inversion input terminal of the comparator 32. The output terminal of the ramp voltage generating circuit 8 is connected to a non-inversion input terminal of the comparator 32.
With the organic LED display device, one field period is divided into a first half scanning period and a second half luminescence period as shown in FIG. 16(a). During the scanning period, the scanning driver applies a scanning voltage to the write transistor TR1 constituting each pixel 31 on each horizontal line, bringing the transistor TR1 into conduction, whereby data voltage is applied to the capacitance element C by the data driver to store the voltage as a charge. As a result, data corresponding to one field is set in all the pixels constituting the LED display device. As shown in FIG. 16(b), the ramp voltage generating circuit maintains a high voltage value during the first half scanning period of every field period and generates during the second half luminescence period thereof a ramp voltage linearly varying from a low voltage value to a high voltage value. During the first half scanning period, the high voltage from the ramp voltage generating circuit is applied to the non-inversion input terminal of the comparator 32. This causes the comparator 32 to always deliver a high output as shown in FIG. 16(c) despite the input voltage to the inversion input terminal thereof. When the circuit applies the ramp voltage to the non-inversion input terminal of the comparator 32 in the second half luminescence period, the output voltage (data voltage) of the capacitance element C is simultaneously applied to the inversion input terminal of the comparator 32. This gives one of two values of high and low as shown in FIG. 16(c) to the output of the comparator 32 in accordance with the result of comparison of the two voltages. Stated more specifically, the output of the comparator is low while the ramp voltage is lower than the data voltage, whereas the output of the comparator is high while the ramp voltage is higher than the data voltage. The length of the period during which the comparator output is low is in proportion to the magnitude of the data voltage. Thus, the output of the comparator 32 is low during a period proportional to the magnitude of the data voltage, whereby the drive transistor TR2 is held on only during this period, holding the EL element 30 on. Consequently, the organic EL element 30 constituting each pixel 31 luminesces only for a period proportional to the magnitude of the data voltage for the pixels 31, within the period of one field, whereby multi-level gradation can be realized.
However, the organic LED display device described has the problem that the organic EL characteristics are shifted due to change with temperature and time of the organic EL element, as shown in FIG. 17, causing a point of operation to be shifted, whereby a luminance is varied. That is, when the organic EL characteristics are shifted rightward due to the change with temperature and time of the organic EL element, current to be passed through the organic EL element decreases, to thereby decrease the luminance, as illustrated. On the other hand, when the organic EL characteristics are shifted leftward, current to be passed through the organic EL element increases, to thereby increase the luminance, as illustrated.
Incidentally, a light emission device is proposed for obtaining a constant luminance, which the device corrects the voltage for a pixel portion so that a drive current to be passed through a light emission element for the entire pixel portion is a reference value calculated from data of a video signal (JP-A No.311898/2002).